Method and system for avoiding freezing of at least one component of a cryogenic fluid inside a cryogenic heat exchanger

Abstract

The invention relates to a method and to a system for avoiding freezing of at least one component of a cryogenic fluid inside a cryogenic heat exchanger by measuring a physical property allowing to indirectly determine the risk of freezing of the least one component of the cryogenic fluid inside the cryogenic heat exchanger.

1 . A method for avoiding freezing of at least one component of a cryogenic fluid stream inside a cryogenic heat exchanger indirectly exchanging heat with a first refrigerant stream circulating inside a closed refrigeration cycle and entering the cryogenic heat exchanger after expansion through at least one expansion means of the closed refrigeration loop to indirectly exchange heat with the cryogenic fluid, the cryogenic fluid stream being different from the first refrigerant stream, and comprising the steps of:
 withdrawing and vaporizing a partial stream of the cryogenic fluid stream which is to be fed into the cryogenic heat exchanger;   measuring at least one physical property of a vaporized partial stream of the cryogenic fluid stream, the at least one physical property measured being an indirect indicator of the risk of freezing of the at least one component of the cryogenic fluid stream inside the cryogenic heat exchanger; Transmitting a measurement of the at least one physical property to computing means;   determining by the computing means if there is a risk of freezing of the at least one component of the cryogenic fluid stream inside the cryogenic heat exchanger based on the transmitted measurement;   if the risk of freezing is confirmed, increasing the temperature of the first refrigerant stream entering the heat exchanger to indirectly exchange heat with the cryogenic fluid stream.   
     
     
         2 . The method according to  claim 1 , wherein the risk of freezing of the at least one component of the cryogenic fluid Is confirmed if the transmitted measurement is within a freezing range calculated by the computing means at the temperature of the first refrigerant stream entering the cryogenic heat exchanger to indirectly exchange heat with the cryogenic fluid. 
     
     
         3 . The method according to  claim 1 , wherein the at least one physical property is chosen from a group comprising the thermal conductivity, the speed of sound, the density, the electrical conductivity, the Wobbe index, the heating value. 
     
     
         4 . The method according to  claim 1 , wherein the temperature of the first refrigerant stream entering the cryogenic heat exchanger to indirectly exchange heat with the cryogenic fluid stream is increased by decreasing the mass flow of the first refrigerant stream circulating within a closed loop and entering the cryogenic heat exchanger to indirectly exchange heat with the cryogenic fluid stream. 
     
     
         5 . The method according to  claim 4 , wherein the mass flow of the first refrigerant stream entering the cryogenic heat exchanger to indirectly exchange heat with the cryogenic fluid is decreased by decreasing the speed of rotation of at least one compressor which forces the first refrigerant stream into the cryogenic heat exchanger, in particular which circulates the first refrigerant stream within the closed refrigeration loop comprising the cryogenic heat exchanger. 
     
     
         6 . The method according to  claim 1 , wherein the temperature of the first refrigerant stream entering the heat exchanger is increased by bypassing around the cryogenic heat exchanger a partial stream of a second refrigerant stream which enters the heat exchanger at a pressure higher than the pressure of the first refrigerant stream, both first and second refrigerant streams being circulated within the same closed refrigeration loop. 
     
     
         7 . The method according to  claim 1 , wherein the pressure drop across the cryogenic heat exchanger on the side of the cryogenic fluid stream is measured, transmitted to the computing means and used for determining the risk of freezing of the at least one component of the cryogenic fluid stream inside the cryogenic heat-exchanger. 
     
     
         8 . A system comprising a cryogenic heat exchanger, a compressor and computing means for avoiding freezing of at least one component of a cryogenic fluid stream inside the cryogenic heat exchanger indirectly exchanging heat with a first refrigerant stream entering the cryogenic heat exchanger to indirectly exchange heat with the cryogenic fluid, comprising:
 a gas conditioning system for vaporizing a partial stream being withdrawn from the cryogenic fluid stream upstream of the heat exchanger   a gas properties transmitter configured to measure at least one physical property of the cryogenic fluid, the at least one physical property being an indirect indicator of the risk of freezing of at least one component of the cryogenic fluid stream inside the cryogenic heat exchanger;   means for transmitting a measurement of the at least one physical property measured by the gas properties transmitter to the computing means configured to determine the risk of freezing of the at least one component of the cryogenic fluid stream inside the cryogenic heat exchanger from the transmitted measurement of the at least one physical property of the cryogenic fluid steam;   means for increasing the temperature of the first refrigerant stream entering the cryogenic heat exchanger to indirectly exchange heat with the cryogenic fluid stream.   
     
     
         9 . The system according to  claim 8 , wherein the at least one physical property measured by the gas properties transmitter is chosen from a group comprising the thermal conductivity, the speed of sound, the density, the electrical conductivity, the Wobbe index, the heating value. 
     
     
         10 . The system according to  claim 8 , wherein the means for increasing the temperature of the first refrigerant stream comprise at least one variable frequency drive for adjusting the speed of rotation of at least one compressor which forces the first refrigerant stream into the cryogenic heat exchanger, in particular which circulates the refrigerant stream inside a closed refrigeration loop comprising the heat exchanger. 
     
     
         11 . The system according to  claim 8 , wherein the means for increasing the temperature of the first refrigerant stream comprise a bypass line with a by-pass valve adapted for bypassing a partial stream of a second refrigerant stream around the cryogenic heat exchanger, both first and second refrigerant streams being circulated within the same closed refrigeration loop. 
     
     
         12 . The system according to  claim 8 , further comprising means for measuring the pressure drop across the cryogenic heat exchanger on the side of the cryogenic fluid stream, the computing means being configured for using the pressure drop across the cryogenic heat exchanger on the side of the cryogenic fluid stream to determine the risk of freezing of the at least one component of the cryogenic fluid stream inside the cryogenic heat exchanger. 
     
     
         13 . A use of the method according to  claim 1  onboard a sea-going LNG carrier. 
     
     
         14 . A ship comprising a system according to  claim 8 . 
     
     
         15 . The ship according to  claim 14 , the ship being a LNG carrier.